[1] Field of the Invention
The present invention relates to a fixing device for fixing unfixed images on a recording medium, and to an image forming apparatus.
[2] Related Art
Image forming apparatuses such as a copier is equipped with a fixing device. One example of the fixing device is a so-called belt nip type fixing device that forms a fixing nip by placing an endless belt in contact with a fixing roller while causing a pressing member to press the endless belt from an inner side thereof against the fixing roller, and then fixes unfixed images (e.g., toner images) formed on a recording medium when the recording medium passes through the fixing nip.
This belt nip type fixing device is composed of (i) a pressing member that presses the endless belt from the inner side thereof against the fixing roller so that an appropriate pressure is applied to the fixing nip, and (ii) a low-friction sheet that is provided between the endless belt and the pressing member. The low-friction sheet is made by coating a surface of its base material with a fluorocarbon resin material or the like. When the low-friction sheet is provided between the pressing member and the endless belt, friction caused by the pressure is small compared to when the pressing member is placed in direct contact with the endless belt.
However, even when the aforementioned low-friction sheet is provided, the pressure applied to form the fixing nip causes the coating (e.g., the fluorocarbon resin material) of the low-friction sheet to wear over time. Under this condition, continuous use of the fixing device will lead to an exposure of the base material of the low-friction sheet, resulting in an increase in the friction and thus causing the drive load of the endless belt to increase. If the rotation of the endless belt becomes unstable due to the increase in the drive load of the endless belt, a difference may arise between the rotation speed of the endless belt and that of the fixing roller. Accordingly, the fixing nip's ability to transport the recording medium decreases, triggering degradation of the image quality such as an image shift.